Thermoplastic composites are usually prepared with the extrusion method, and straw reinforcement material must be processed to fiber or powder. In this study, film-roll hot pressing was developed to reinforce linear low density polyethylene (LLDPE) with long continuous straw stems. The long straw stems were wrapped with LLDPE film and then hot pressed and cooled to prepare straw/LLDPE composite. Extruded straw fiber/LLDPE composite was prepared as a control. The mechanical properties of these LLDPE-based composites were evaluated. The hot pressed straw/LLDPE composite provided higher tensile strength, tensile modulus, flexural strength, flexural modulus, and impact strength than the traditional extruded straw/LLDPE composite, by 335%, 107%, 68%, 57%, and 181%, respectively, reaching 35.1 MPa, 2.65 GPa, 3.8 MPa, 2.15 GPa, and 25.1 KJ/m 2 . The density of the hot pressed straw/LLDPE composite (0.83 g/cm 3 ) was lower than that of the extruded straw/LLDPE composite (1.31 g/cm 3 ), and the former had a higher ratio of strength-to-weight. Scanning electron microscopy indicated that the orientation of the straws in the composite was better with the new method. Differential scanning calorimetry tests revealed that in hot pressed straw/LLDPE composite, straw fibers have a greater resistance to the melting of LLDPE than extruded composite. Rotary rheometer tests showed that the storage modulus of the hot pressed straw/LLDPE was less affected by frequency than that of the extruded composite, and the better elastic characteristics were pronounced at 150 • C. The hot pressed straw/LLDPE composite absorbed more water than the extruded composite and showed a potential ability to regulate the surrounding relative humidity. Our results showed that straw from renewable sources can be used to produce composites with good performance.Researchers have made some achievements in straw fiber reinforced composites. Nyambo et al. [7] used maleic acid-grafted polyurethane (PU-g-MA) to improve the interfacial adhesion between wheat straw and polyurethane (PU). They found that the addition of 3 phr and 5 phr PU-g-MA significantly increased the tensile strength (20%) and flexural strength (14%) of straw/PU composites, and proved that the increase in strength was due to the well combination of fibers and matrix. Xiao et al.[8] treated the straw with NaOH solution, blended the straw, polyethylene, stearic acid and maleic anhydride, then hot pressed to manufacture the straw/PP composite. The composite has low water absorption and good acid and alkali resistance; Zhang et al. [9] investigated the effects of different straw treatment methods, the particle size of straw powder, and the mass fraction of straw on the mechanical properties of straw/PP composites. The results show that when the straw is treated with the silane coupling agent KH570, the mechanical properties of the straw/PP composite are the best when the particle size of the straw powder is 60 mesh and the mass fraction is 50%; Zabihzadeh et al. [10] investigated the effect of maleic ...
To reduce the pollution resulting from discarding waste plastic film and burning straw, a new method of preparing straw-reinforced LLDPE composites was developed to utilize these wastes. The straws were first laid parallel on an LLDPE film and then rolled up. The rolls containing long straws were laid into a mat and then hot-pressed into a long straw composite board (the mass of straw accounted for 60%). Slope-cutting the straw, grinding the straw, and twisting the roll were designed to improve the physical and mechanical properties of long straw composites. Among them, slope-cutting the straw combined with twisting the roll provided the best properties. Compared to the extruded straw particle composite, the composite prepared with the new method improved the tensile strength, bending strength, impact strength, and water resistance by 358%, 151%, 416%, and 81%, respectively. Slope-cutting exposed more inner surface at the end of the straw. Scanning electron microscope observations showed that the straw inner surface was more tightly bonded with the LLDPE matrix than the outer surface. Meanwhile, the integrity of the straw was retained as much as possible, and thus greatly improved the performance of the resulting composites. Dynamic mechanical analysis, differential scanning calorimetry, and thermogravimetric analysis show that the viscous deformation of the composites prepared by the new method was reduced and the rigidity was increased, and the combination of straw and LLDPE forms a dense composite material with good interfacial bonding. It greatly slowed down the degree of its pyrolysis.
Straw utilization is a key issue related to agricultural production and air pollution control. In this study, a novel extrusion process was proposed to improve the physical and mechanical properties of the straw-reinforced linear low-density polyethylene (LLDPE) composite. Instead of crushing the straw and mixing it with plastic matrix, the new method mixes straw with plastic matrix in its original form. The intact long rice straws were parallelly spread on the LLDPE film and then rolled up together into a prefabricated roll. The rolls experienced three extrusion processes as follows: (1) twin-screw melting, cooling and crushing, single-screw extruding; (2) twin-screw melting and single-screw extruding; (3) directly single-screw extruding. The testing results showed that the straw/LLDPE composite (with a ratio of 6:4) prepared by Method (2) exhibited optimized properties. Characterization by scanning electron microscopy indicated that the damage to rice straw fibers was relatively minor, the orientation of long fibers was good, and the binding of fibers with the LLDPE matrix was excellent in this case. The results of dynamic mechanical testing (DMA), differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis demonstrated that composites prepared by the new process exhibited significantly improved thermal stability and energy storage modulus, compared with those prepared by conventional processes (e.g., extruded straw particles/LLDPE composite). The new proposed method yielded significantly enhanced mechanical properties while reducing dust pollution.
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